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Toxocariasis

Parasite Biology

Causal Agents

Toxocariasis is caused by larvae of Toxocara canis (dog roundworm) and less frequently of T. cati (cat roundworm), two nematode parasites of animals.

Life Cycle

Toxocara canis accomplishes its life cycle in dogs, with humans acquiring the infection as accidental hosts. Unembryonated eggs are shed in the feces of the definitive host . Eggs embryonate and become infective in the environment . Following ingestion by dogs , the infective eggs hatch and larvae penetrate the gut wall. In younger dogs, the larvae migrate through the lungs, bronchial tree, and esophagus; adult worms develop and oviposit in the small intestine . In older dogs, patent infections can also occur, but larval encystment in tissues is more common. Encysted stages are reactivated in female dogs during late pregnancy and infect by the transplacental and transmammary routes the puppies , in whose small intestine adult worms become established . Puppies are a major source of environmental egg contamination. Toxocara canis can also be transmitted through ingestion of paratenic hosts: eggs ingested by small mammals (e.g. rabbits) hatch and larvae penetrate the gut wall and migrate into various tissues where they encyst . The life cycle is completed when dogs eat these hosts and the larvae develop into egg-laying adult worms in the small intestine. Humans are accidental hosts who become infected by ingesting infective eggs in contaminated soil or infected paratenic hosts . After ingestion, the eggs hatch and larvae penetrate the intestinal wall and are carried by the circulation to a wide variety of tissues (liver, heart, lungs, brain, muscle, eyes) . While the larvae do not undergo any further development in these sites, they can cause severe local reactions that are the basis of toxocariasis. The two main clinical presentations of toxocariasis are visceral larva migrans and ocular larva migrans. Diagnosis is usually made by serology or the finding of larvae in biopsy or autopsy specimens.

Geographic Distribution

Worldwide.

Clinical Presentation

Many human infections are asymptomatic, with only eosinophilia and positive serology. The two main clinical presentations of toxocariasis are visceral larva migrans (VLM) and ocular larva migrans (OLM). In VLM, which occurs mostly in preschool children, the larvae invade multiple tissues (liver, heart, lungs, brain, muscle) and cause various symptoms including fever, anorexia, weight loss, cough, wheezing, rashes, hepatosplenomegaly, and hypereosinophilia. Death can occur rarely, by severe cardiac, pulmonary or neurologic involvement. In OLM, the larvae produce various ophthalmologic lesions, which in some cases have been misdiagnosed as retinoblastoma, resulting in surgical enucleation. OLM often occurs in older children or young adults, with only rare eosinophilia or visceral manifestations.

Image Gallery

Toxocara sp. eggs.

Eggs are not clinically diagnostic for human cases. Humans are paratenic hosts for Toxocara spp. and eggs are found only in the feces of definitive hosts (cats and dogs). Toxocara eggs are subspherical, thick-shelled and have a pitted surface. The size range for the three most commonly observed species differs slightly: T. canis measure 80-85 micrometers by 75 micrometers; T. cati measure 65-75 micrometers; and T. leonina measure 75-85 micrometers by 60-75 micrometers.

Figure A: Toxocara sp. egg teased from an adult worm. The worm was never identified, but the egg size is most consistent with T. cati. Image courtesy of the New Jersey State Public Health Laboratory.

Figure B: Toxocara sp. egg teased from an adult worm. The worm was never identified, but the egg size is most consistent with T. cati. Image courtesy of the New Jersey State Public Health Laboratory.

Figure C: Toxocara sp. eggs teased from an adult worm and stained with iodine, magnification at 100×. The adult worm was never identified in this case. Image courtesy of the Alaska State Public Health Laboratory.

Figure D: Egg from same specimen as C but at 400× magnification. Object measured approximately 75 micrometers.

Toxocara canis larva hatching.

Figure A: Toxocara canis larva beginning to hatch.

Figure B: T. canis larva hatching.

Figure C: T. canis larva.

Adult Toxocara sp. worms.

Adult Toxocara spp. measure approximately 4-6 cm long (males) and 6-10 cm long (females). Like most ascarids, Toxocara have three “lips” on the anterior end of the worm. They also possess large cervical alae with striations.

Laboratory Diagnosis

Laboratory Diagnosis

In this parasitic disease the diagnosis does not rest on identification of the parasite. Since the larvae do not develop into adults in humans, a stool examination would not detect any Toxocara eggs. However, the presence of Ascaris and Trichuris eggs in feces, indicating fecal exposure, increases the probability of Toxocara in the tissues. For both VLM and OLM, a presumptive diagnosis rests on clinical signs, history of exposure to puppies, laboratory findings (including eosinophilia), and the detection of antibodies to Toxocara.

Antibody Detection

Antibody detection tests are the only means of confirmation of a clinical diagnosis of visceral larva migrans (VLM), ocular larva migrans (OLM), and covert toxocariasis (CT), the most common clinical syndromes associated with Toxocara infections. The currently recommended serologic test for toxocariasis is enzyme immunoassay (EIA) with larval stage antigens extracted from embryonated eggs or released in vitro by cultured infective larvae. The latter, Toxocara excretory-secretory (TES) antigens, are preferable to larval extracts because they are convenient to produce and because an absorption-purification step is not required for obtaining maximum specificity. Evaluation of the true sensitivity and specificity of serologic tests for toxocariasis in human populations is not possible because of the lack of parasitologic methods to detect Toxocara parasites. These inherent problems result in underestimations of sensitivity and specificity. Evaluation of the Toxocara EIA in groups of patients with presumptive diagnoses of VLM or OLM indicated sensitivity of 78% and 73%, respectively, at a titer of >1:32. When the cutoff titer for OLM cases was lowered to 1:8, sensitivity was increased to 90%. Further confirmation of the specificity of the serologic diagnosis of OLM can be obtained by testing aqueous or vitreous humor samples for antibodies. Specificity has been reported to be >90% at a titer of >1:32. When interpreting the serologic findings, clinicians must be aware that a measurable titer does not necessarily indicate current clinical Toxocara canis infection. In most human populations, a small number of those tested have positive EIA titers that apparently reflect the prevalence of asymptomatic toxocariasis. In the United States The age adjusted seroprevalence rate was estimated to be approximately 14%, using sera from over 20,000 participants in the Third National Health and Nutrition Examination Survey (1988–1994).